Metal-Insulator-Metal (MIM) capacitors have been widely used in functional circuits such as mixed signal circuits, analog circuits, Radio Frequency (RF) circuits, Dynamic Random Access Memories (DRAMs), embedded DRAMs, and logic operation circuits. In system-on-chip applications, different capacitors for different functional circuits have to be integrated on a same chip to serve different purposes. For example, in mixed-signal circuits, capacitors are used as decoupling capacitors and high-frequency noise filters. For DRAM and embedded DRAM circuits, capacitors are used for memory storage, while for RF circuits, capacitors are used in oscillators and phase-shift networks for coupling and/or bypassing purposes. For microprocessors, capacitors are used for decoupling. The traditional way to combine these capacitors on a same chip is to fabricate them in different metal layers.
Decoupling capacitors are used to decouple some parts of electrical networks from others. Noise caused by certain circuit elements is shunted through the decoupling capacitors, hence reducing the effect of the noise-generating circuit elements on adjacent circuits. In addition, Decoupling capacitors are also used in power supplies, so that the power supplies may accommodate the variations in current-draw, so that the variation in power supply voltage is minimized. When the current-draw in a device changes, the power supply itself cannot respond to the change instantaneously. The decoupling capacitors thus may act as power storages to maintain power supply voltages in response to the current-draw at frequencies ranging from hundreds of kilo-hertz to hundreds of mega-hertz.